Although a historic downturn in PC shipments has made headlines since April, "peak PC" -- the moment when personal computers crested -- was two years ago, according to data from researcher IDC.

In the third quarter of 2011, global PC shipments totaled 96.1 million, slightly more than the following quarter's 95.8 million, IDC said. But shipments declined 4 percent in 2012 and are headed for an even larger dip of 8 percent, perhaps more, in 2013.

"It's very possible that that was the peak," said Bob O'Donnell, an analyst at IDC who is on the PC shipment tracking team, referring to the third quarter of 2011.

What's not clear is whether PCs are on an irrevocable slide or will reach a lower-level plateau and stabilize. O'Donnell believes the latter. "I do think that [the industry] can come up from where it is today," he said.

Lost momentumAl Gillen, also with IDC, and one of several analysts there who cover Microsoft, agreed. "We're not predicting a continual contraction, the PC business has not gone into a free fall where years out it vanishes," said Gillen. "But what has happened, it's lost momentum."

Even so, IDC, which had predicted that the second half of the year would salvage the year from an even steeper decline and set the stage for a minor comeback in 2014, is now reconsidering. Its research rival Gartner sees the same trend.

O'Donnell said that the firm now suspects that the third and fourth quarters of this year will play out worse than previously anticipated. "The numbers are TBA," said O'Donnell.

Both IDC and Gartner are now convinced that 2014 will be the signal year for PCs. If shipments recover next year, 2013 could prove to be the rock bottom. Additional declines throughout 2014, however, could be debilitating for any business that overwhelmingly relies on the traditional desktop-plus-notebook model, especially Microsoft, whose Windows and Office depend on new PCs to drive growth.

Cautious optimismIDC and Gartner remain cautiously optimistic that 2014 will be better for the PC business. Less than a month ago, Gartner pegged total PC shipments in 2014, including a separate category it labeled "ultramobile," at 329 million, or 1.1 percent higher than 2013's estimate.

Likewise, O'Donnell saw a silver lining in the second quarter's U.S. numbers, which were down just 1 percent year-over-year. "The U.S. is a bellwether, and we started to see some turnaround there last quarter," he noted. In the United States, some of the improved numbers were driven by corporate upgrades from the soon-to-retire Windows XP, a process O'Donnell said could generate sales past XP's April 2014 end-of-life as many companies fail to meet the deadline.

But other experts disagreed with IDC's and Gartner's measured confidence that 2013 will be the toughest year the PC industry sees.

"Analyst firms have been expecting the PC industry to rebound for more than a year now," said Sameer Singh, an analyst who writes on the Tech-Thoughts blog. "I don't expect that to happen as the reason for this decline is an irreversible, secular shift in computing patterns."

From PCs to tabletsThe shift Singh referred to was the one from traditional PCs to tablets. And in some cases -- particularly in emerging markets where PC ownership has never gained a foothold -- the pattern entirely abandons the idea of personal computers in favor of smartphones, and then as the logical upgrade, tablets.

Tablet shipments, said Singh, may outstrip PCs as early as the third quarter of this year, and no later than the fourth quarter.

Singh dubbed the trend "tablet-PC replacement rate," and believes it points to the real possibility that PC shipments will, rather than uptick at some point, continue to contract, perhaps to a point where they average between 65 and just over 70 million units per quarter. If accurate, that would return the PC industry to shipment volumes of 2007-2008, and represent as much as a 28 percent contraction from peak PC.

O'Donnell acknowledged the crystal ball remain cloudy, but said it's certainly possible that the PC business might settle at 20 percent below peak PC, or around 290 million systems shipped annually. "That could be the level, stable reality," O'Donnell said.

Neither O'Donnell nor Gillen was convinced that PCs would disappear or mutate to the point where they become unrecognizable, making the point -- long argued by Microsoft and Apple, among others -- that there will remain tasks that more-or-less traditional PCs are better suited to handle than tablets.

"As people spend more and more time with tablets, I think they will come to realize that [tablets] are not really a PC replacement," said O'Donnell, taking the opposite side of the debate from Singh.

Longer replacement cyclesO'Donnell's faith in the survival of the PC, albeit at a reduced rank, is based on the belief that while replacement cycles have lengthened -- as users let more time go by before buying a new personal computer -- those cycles will eventually stabilize.

Of all technology companies, peak PC has to be foremost in Microsoft's mind. That explains the urgency in Microsoft's shift to, in the refrain of CEO Steve Ballmer, a "devices and services" business model.

For Gillen, the de-emphasis of the Windows client under Microsoft's new strategy is a logical reaction to Peak PC. "The lost momentum [of PC shipment growth] means Microsoft was not able to grow its Windows client business," said Gillen, a big problem for a publicly traded company where financials are scrutinized every quarter. "But by putting the operating system businesses together, Microsoft can essentially offset the decline of the PC client with gains coming from mobile. It's a more holistic view."

Gillen was referring to Microsoft's announced reorganization, which will combine Windows 8, Windows RT, and Windows Phone -- but not Windows Server -- into a single unit: the Operating Systems Engineering Group, headed by Windows Phone boss Terry Myerson.

Microsoft has timeAnd Microsoft has time to make the strategy work, Gillen maintained. "People are not going to suddenly stop using Windows," he said. "Windows will be around as long as PCs continue to sell. This takes some of the pressure off so they can focus on the longer-term [devices and services] strategy."

While Ballmer obviously never breathed "peak PC" in his memos last week, or even mentioned the decline in PC shipments in those missives or in a conference call with Wall Street, he seems to get the dilemma. Why else would he hammer home "family of devices" as the firm's new tune, if not to replace what's been lost as PC shipments slide?

But getting it doesn't mean doing it, Gillen cautioned. "If PC shipments continue to show softness, it's going to require some very fancy footwork," said Gillen. "This is going to be a tough shift for Microsoft to make, to ask them to now accept that the world is a very different place."